A present participle becomes a gerund - i.e. a noun - when it acts as the subject or the object of a verb. To take your example of typing:

1. Typing IS an efficient way of getting words onto a page. (SUBJECT)
2. Typing quickly MAY CAUSE a typist to make mistakes. (SUBJECT)
3. Inexperienced typists should LEARN typing. (OBJECT)
4. This computer program TEACHES typing. (OBJECT)

Now look at this example: Swimming (SUBJECT) IS a very complete sport, but some people PREFER running, cycling, or skiing. (3 OBJECTS)

In the following sentences, TYPING is a present participle. It is not the subject or doer of the action, but rather is itself part of the action.

1. Typing quickly, the secretary finished the letter.
2. People often used to smoke while they were typing.

There is more that could be said on this matter, but the above is a simple resumé.

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